When a plurality of base stations, each covering an area of hundreds of meters or several kilometers with high transmit power (hereinafter called a macro-cell base station), are installed for a radio communication system, a mobile station can make radio communications in a wide area. However, since radio waves used for radio communications may be intercepted or may be attenuated by a building or the like, there may be places in which radio waves from a macro-cell base station weaken, for example, indoor places.
In some cases, a base station which uses low transmit power and covers a small area (hereinafter called a femto-cell base station) is installed in order to ensure stable communications even in a place where radio waves from a macro-cell base station weaken. When a femto-cell base station is installed, a mobile station can make communications even in a place where radio waves from the macro-cell base station weaken.
On the other hand, in a cellular radio communication system, since a plurality of cells share radio resources, a communication in a cell causes interference to a communication in another cell. For example, since a downlink communication in a cell from a base station to a mobile station results in interference to a downlink communication in an adjacent cell, the quality of signals received by a mobile station near a cell boundary may be deteriorated. Also, an uplink communication in a cell from a mobile station to a base station results in interference town uplink communication in an adjacent cell. Therefore, in order to ensure a satisfactory communication quality, the mobile station must increase the power for uplink transmission, resulting in an increase in power consumption by the mobile station. Unless the uplink transmit power is increased so as to compensate for the effect of interference to an uplink communication, the quality of uplink signals received by the base station is deteriorated.
In order to reduce the effect of such interference, JP-A-2009-253569 discloses a technique which reduces the interference between a macro-cell base station and a femto-cell base station. Also, JP-A-2007-243505 discloses a technique which reduces the interference to other cells even when a small radio base station device is relocated.